FIG. 15 is a block diagram of a conventional hands-free system. A cellular phone base station 1 and a cellular phone 2 are connected via a public radio communication line. Signals are transmitted and received between the cellular phone 2 and a hands-free unit 3 via a connector 2C prepared in the cellular phone 2. A microphone 4 and a speaker 5 are connected to the hands-free unit 3. Telephone signals gathered by the microphone 4 arranged in a vehicle are transmitted to the cellular phone base station 1 via the hands-free unit 3 and the cellular phone 2. Then, the telephone signals are transmitted from the cellular phone base station 1 to a person on the other end of a so-called public telephone line. On the other hand, telephone signals from the person on the other end of the line are transmitted to the hands-free unit 3 through the reverse path. The speaker 5 reproduces the telephone signals from the person on the other end of the line inputted to the hands-free unit 3. The cellular phone 2, the hands-free unit 3, the microphone 4 and the speaker 5 are arranged in the vehicle 6.
The hands-free system shown in FIG. 15 has a disadvantage in the aspect that it is necessary to connect the connector 2C of the cellular phone 2 every time a cellular phone user gets on the vehicle. Moreover, it is also necessary to disconnect the connector 2C when the user gets out of the vehicle. Furthermore, when the connector 2C is not connected, the connector 2C with cable should be put away to avoid the car room from looking not neat.
FIG. 16 is a block diagram of another conventional hands-free system, which overcomes the mentioned disadvantage of the hands-free system of FIG. 15. The hands-free system shown in FIG. 16 is disclosed, for example, in the Japanese Patent Publication (unexamined) No. 276261/1998 titled “Radio Communication System”. A difference from the hands-free system of FIG. 15 consists in that the, instead of the cable system, the cellular phone 2a and the hands-free unit 3a are connected by a radio system (or an infrared system). Flow of the telephone signals is the same as in the case of FIG. 15, and further description thereof is omitted herein. In another known system disclosed in the Japanese Patent Publication (unexamined) No. 331064/1999 titled “Hands-free Telephone System”, a radio (FM) circuit portion built in the cellular phone 2a body is formed into a radio module 7 and is arranged at a connector portion of the cellular phone 2a. 
The disadvantages of connecting and disconnecting the connector of the cellular phone 2a and looking not neat are overcome by using the radio system (or the infrared system) in connecting the cellular phone 2a and the hands-free unit 3a. In the case of the infrared system, it is necessary that infrared transmitting and receiving sections of the cellular phone 2a and the hands-free unit 3a are arranged opposite to each other. On the contrary, in the case of the radio system, it is not necessary that the transmitting and receiving sections of the cellular phone 2a and the hands-free unit 3a are arranged opposite to each other.
As a short-range radio communication system of such type, Bluetooth SIG has proposed a “Bluetooth” system. (BLUETOOTH is a registered trademark of Telefonaktiebolaget L M Ericsson, Sweden.)
The mentioned system has the following characteristics:    (1) Cordless communication between one apparatus and another is achieved.    (2) Radio circuit of this system is applicable to a variety of apparatuses such as personal computers, printers, cellular phones, personal digital assistants, etc.    (3) The global standard is adopted.    (4) Both voice and data communication can be transmitted and received.    (5) A frequency band of 2.4 GHz called an ISM band is adopted.    (6) It is not necessary to arrange the communication apparatuses opposite to each other unlike those in the infrared communication.    (7) Communication between apparatuses is available in the form of not only 1 to 1 like the infrared communication but also 1 to 7 (maximum).    (8) It is possible to choose any output power convenient for each apparatus because there are following three types of outputs:    Class 1: 100 mW (+20 dBm) max;    Class 2: 2.5 mW (+4 dBm) max; and    Class 3: 1 mW (0 dBm) max.
In choosing one output power among the three types described in the foregoing characteristic (8) of the system, Class 3 of the smallest power consumption (i.e., the battery life is long) for a portable device such as cellular phone. It is preferable to adopt Class 1 from the viewpoint of giving an importance to a communication area such as home modem station where communication with each room is available on the power-saving basis.
Supposing that a radio communication is conducted using a cellular phone having a small radio communication output power function like in the foregoing system with a cellular phone brought into a vehicle, it is possible to utilize this radio communication enjoying the following advantages: p0 (1) Hands-free telephone    {circle around (1)} Cordless and hands-free telephone is available even when the cellular phone is put in a bag, on a rear seat, or the like.    {circle around (2)} This contributes to safe driving.    (2) Information and communication display    {circle around (1)} Information provided by an IT-adapted cellular phone of a mobile communication company is displayed on a map display of a car navigation unit. The information is enlarged on the display and easy to see.    {circle around (2)} Plural persons in the vehicle can see the display at the same time.    {circle around (3)} This contributes to safe driving.
Now, environment of the radio communication is described below.    (1) A power-saving type is adopted as the radio output power of the cellular phone in most cases.    {circle around (1)} In the case of power-saving output power, it is considered that communication is available within a distance of several meters.    (2) In the cases of the foregoing hands-free telephone (1) and information and communication display (2), a radio communication circuit is built in a car navigation unit in most cases.    {circle around (1)} As the car navigation unit is composed of a sheet metal chassis, a radio communication antenna is arranged inside the sheet metal chassis of the car navigation unit, and therefore sensitivity is low.    {circle around (2)} The car navigation unit is arranged at any of various places such as center console, under side of seat, and trunk.    {circle around (3)} In the case of arranging the car navigation unit in the trunk, to be free from outside influence such as “pressure”, “waterdrop”, and the like from loaded cargo, the unit incorporating the communication antenna of the car navigation may be further covered with a sheet metal when required.
Accordingly, an object of the invention is to provide a short-range on-vehicle radio communication system having in-car radio communication function, capable of widening the communication area so that any signal from a cellular phone of low radio communication output power as described above may be easily received.